At idle there is no significant load on the torque converter, so changing it into neutral won't make a difference, apart from wearing out the linkage earlier. If you are stopped for more than 15 seconds, it makes more sense to turn the engine off, as that will reduce wear and save fuel (assuming you are in a location where that is safe to do so).

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue